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I have found this attached image of the waveform of the voltage across the injector in the D-Jetronic system fitted to my Citroen. The description of the waveform and why it has that shape is given below.
The description seems to say the drive transistors are simply switched on and it is the inductance of the injector that controls the waveform.
Do I need to do more that set injtrig?
"The nominal specification value of Rinj is 2.4 ohms. Measurements I've taken on one of my injectors show Linj to be 3.77 mH and Rinj 2.69 ohms. When the driver transistor turns on and current begins to flow through the injector, a magnetic field begins to build in the coil and starts to pull the injector open. Due to the energy required to develop the magnetic field, the inductive reactance (XL - the effective resistance of the inductor to a time-varying signal, measured in ohms) is initially very large. Because XL is initially the largest resistance in the series path of circuit elements (Rload, Rinj, and XLinj), almost all of the voltage is dropped across the injector - that's why right after the injector turns on you see +12.7 V across it.
The rate the magnetic field builds is governed by the time constant, t = Linj / R, where R = Rinj. Therefore, t = 0.00377 / 2.69 = 1.40 msec. As the field builds, XL decreases and less voltage is dropped across the injector - that's why the voltage in the waveform below decays from the peak of +12.7 V. It takes about 2.5 * t for the decay to be complete, about 3.5 msec. Note that this means that for injection pulses shorter than this duration, the magnetic field is still decaying when the injector is turned off. Once the magnetic field is at steady-state, the voltage drop across the inductor is due solely to I * Rinj, where I is the steady-state current. If you use the nominal spec value of R = 2.4 ohms and an assumed battery voltage of +12 V, you would get a sustained voltage drop across the injector of 3 V. When the car is running, the actual system voltage is more like 13.6 V. Accounting for the higher measured injector resistance over the specification value and using the system voltage of 13. 6 V, the actual measured sustained voltage is 4 to 5 V. Note that the calculated drive voltage corresponds exactly with data given for the flow rates of the injectors, which states that the tests were performed at 3 V (data courtesy of Roland Kunz).
At the end of the injection period, the driver transistor turns off, which rapidly changes the current drive to the injector to zero. The rapid collapse of the magnetic field in the coil now has an opposite effect - it induces a high negative voltage across the inductor. The mathematical expression of this voltage is VL = Linj * dI/dt , where dI/dt is the time derivative of current through the inductor. The net effect is that you get a large negative spike of voltage after the injector turns off. I have measured a peak of -27 V, decaying in about 1 msec to zero volts. The inductive voltage spike (and resultant current) is limited by the series combination of a 6.8 mF capacitor and load resistor in parallel with the injector. Note that since D-Jetronic is a grouped injection system, a paired set of injectors experience all of the events described above simultaneously. When the injectors turn off, the inductive voltage spike of both injectors is limited by the 6.8 mF capacitor and load resistor. "
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